[tasks] Add a new background helper 'tasks' extension.

docs/ext/tasks/index.rst

@@ -0,0 +1,82 @@
+``discord.ext.tasks`` -- asyncio.Task helpers
+====================================================
+
+One of the most common operations when making a bot is having a loop run in the background at a specified interval. This pattern is very common but has a lot of things you need to look out for:
+
+- How do I handle :exc:`asyncio.CancelledError`?
+- What do I do if the internet goes out?
+- What is the maximum number of seconds I can sleep anyway?
+
+The goal of this discord.py extension is to abstract all these worries away from you.
+
+Recipes
+---------
+
+A simple background task in a :class:`~discord.ext.commands.Cog`:
+
+.. code-block:: python3
+
+    from discord.ext import tasks, commands
+
+    class MyCog(commands.Cog):
+        def __init__(self):
+            self.index = 0
+            self.printer.run()
+
+        def cog_unload(self):
+            self.printer.cancel()
+
+        @tasks.loop(seconds=5.0)
+        async def printer(self):
+            print(self.index)
+            self.index += 1
+
+Adding an exception to handle during reconnect:
+
+.. code-block:: python3
+
+    import asyncpg
+    from discord.ext import tasks, commands
+
+    class MyCog(commands.Cog):
+        def __init__(self, bot):
+            self.bot = bot
+            self.data = []
+            self.batch_update.add_exception_type(asyncpg.PostgresConnectionError)
+            self.batch_update.run()
+
+        def cog_unload(self):
+            self.batch_update.cancel()
+
+        @tasks.loop(minutes=5.0)
+        async def batch_update(self):
+            async with self.bot.pool.acquire() as con:
+                # batch update here...
+                pass
+
+Looping a certain amount of times before exiting:
+
+.. code-block:: python3
+
+    from discord.ext import tasks
+
+    @tasks.loop(seconds=5.0, count=5)
+    async def slow_count():
+        print(slow_count.current_loop)
+
+    slow_count.run()
+
+Doing something after a task finishes is as simple as using :meth:`asyncio.Task.add_done_callback`:
+
+.. code-block:: python3
+
+    afterwards = lambda f: print('done!')
+    slow_count.get_task().add_done_callback(afterwards)
+
+API Reference
+---------------
+
+.. autoclass:: discord.ext.tasks.Loop()
+    :members:
+
+.. autofunction:: discord.ext.tasks.loop